1. Field of the Invention
The present invention describes the use of zinc treated with metal hydride in organometallic synthesis.
2. Background of the Invention
The still unpublished European application 00102815.8 discloses the reaction of metallic zinc with metal hydrides, in particular magnesium hydride. The relevant contents of this patent application are hereby fully incorporated by reference.
Previous fields of application of hydrides in metallurgy or materials science are restricted mainly to use for producing metal foams (for example TiH2 or MgH2 as blowing agent for aluminum foams; U.S. Pat. No. 5,972,285; DE-C41 01 630; EP-B-0 559 097), production of microcrystalline or amorphous metals or alloys by reduction of corresponding metal salts by means of hydrides (U.S. Pat. No. 5,951,739; DE-C-3 934351; Bxc3x6nnemann, Brijoux, Joussen, Angew. Chem., 1990, 102, 324-326), solid-state phase transformations of alloys via hydride formation by treatment with hydrogen (Sakamoto, Chen, Ura, Flanagan, Ber. Bunsen-Ges. 1995, 99, 807-820), decarburization of steel by means of hydrides (JP-C-05 287 361) and the activation of aluminum by means of alkali metal hydrides or alkaline earth metal hydrides for the synthesis of aluminum alkyls (Houben-Weyl, Methoden der organischen Chemie, 1970, volume 13/4, 33; U.S. Pat. No. 2,989,557; U.S. Pat. No. 3,050,541; BE-C-559 404; U.S. Pat. No. 3,077,490).
In the field of organic chemistry, organometallic zinc reagents open up important routes to a series of pharmaceuticals (for example steroids), derivatives of natural products (for example terpenes, fatty acids) and insecticides (for example pyrethroids).
As representative examples of the use of zinc in organometallic reactions, its use in Simmons-Smith and Reformatsky reactions will be described here.
In Simmons-Smith reactions, olefins are reacted with diiodomethane and metallic zinc, typically in diethyl ether as solvent, to form cyclopropanes. According to the literature (Houben-Weyl, Methoden der organischen Chemie, 1973, volume 13/2a, 838-852, herein incorporated by reference), CH2I2 and Zn first form the insertion product ICH2ZnI, as reactive intermediate, and this subsequently transfers carbene CH2 to the double bond of the olefin used with elimination of ZnI2 in a single-stage mechanism.
In Reformatsky reactions, xcex1-halo esters are reacted with metallic zinc and aldehydes or ketones to give xcex2-hydroxycarboxylic esters. In the first step, according to the literature (Houben-Weyl, Methoden der organischen Chemie, 1973, volume 13/2a, 809-838, herein incorporated by reference), the zinc inserts into the halogen-carbon bond of the xcex1-halo ester to form a nucleophilic organozinc intermediate which in the second step reacts with the electrophilic carbonyl carbons of aldehydes or ketones to give xcex2-hydroxycarboxylic esters.
In the two types of reaction described, a very high product yield requires not only suitable reaction conditions (type of solvent, temperature, choice of concentrations, reaction time, etc.) but also appropriate pretreatment or activation of the metallic zinc used. To increase the reactivity (destruction of the oxide layer, achievement of a particularly fine division) of zinc in Simmons-Smith and Reformatsky reactions, numerous physical methods (for example mechanical milling, use of ultrasound, metal vaporization, electrolytic ultra-high purification) and chemical methods (for example Rieke processes, zinc-copper couple formation, etching processes, potassium-graphite processes) have been developed in the past (A. Fxc3xcrstner, Active Metals, Preparation, Characterization, Applications, VCH, 1996).
As a criterion for the quality or efficiency of the physical or chemical activation method employed in each case, the use of the respective treated metallic zinc can be tested in organometallic synthetic chemistry, for example in Simmons-Smith and Reformatsky reactions, by determining the product yield in each case.
Simmons and Smith (J. Am. Chem. Soc., 1958, 80, 5323-5324; J. Am. Chem. Soc., 1959, 81, 4256-4264) themselves describe what they call the cyclopropanation reaction using a zinc-copper couple which was obtained in a relatively complicated fashion by reduction of a copper oxide/zinc mixture in a stream of hydrogen at 500xc2x0 C. Later work by other authors repeatedly emphasizes the problems of a good and reproducible method of preparing the zinc-copper couple and propose alternative methods (for example Shank, Shechter, J. Org. Chem, 1959, 24, 1825-1826; LeGoff, J. Org. Chem., 1964, 29, 2048-2050). The simplest method of preparation may well be that descried by Rawson and Harrison (J. Org. Chem., 1970, 35,2057-2058).
They produce the zinc-copper couple by heating a suspension of zinc containing 10% of copper(I) chloride for 30 minutes. More recent work on alternatives to the zinc-copper couple include, for example, the use of electrolytically ultra-highly purified zinc (Takai, Kakiuchi, Utimoto, J. Org. Chem., 1994, 59, 2671-2673), the use of zinc activated by means of small amounts of elemental iodine in steroid synthesis (DD-C-143 782) or the use of catalytic amounts of hydride complexes such as sodium bis(2-methoxyethoxy)aluminum hydride (Vitride(copyright)) in the cyclopropanation of unsaturated fatty acid esters (U.S. Pat. No. 4,472,313).
For use of zinc in Reformatsky reactions, a similar type and variety of activation methods as described above for Simmons-Smith reactions is employed. Simple activation by means of reagents such as iodine, dibromomethane, etc., or washing with dilute mineral acids frequently gives only moderate success. Once again, the use of specific alloys (for example zinc-copper or zinc-silver couples) is substantially more effective. A very effective chemical activation method is the reduction of zinc halides by potassium in the Rieke process. Further methods include reduction using potassium-graphite or the use of ultrasound (Fxc3xcrstner, Angew. Chem., 1993, 105, 171-197; Rieke, Uhm, Synthesis, 1975, 452-453; Han, Boudjouk, J. Org. Chem., 1982, 47,5030-5032).
All the abovementioned activation methods for zinc have the disadvantage that they are either relatively complicated and costly or have only very restricted applicability or effectiveness, or lead to unreproducible yields. The industrial problem of an efficient and economical way of carrying out further organozinc reactions which are not encumbered by the abovementioned disadvantages has thus not been solved hitherto.
It has surprisingly been found that treatment of molten zinc with metal hydrides produces zinc which overcomes the abovementioned disadvantages extremely well and can be employed directly and efficiently in organometallic syntheses without using the above-described activation methods of the prior art.
According to the invention, it has been found that commercially available metallic zinc which is heated together with about 2% by weight of metal hydride (for example LiH, MgH2, AlH3, TiH2, etc.) or metal hydride mixtures to the melting point and then cooled to room temperature and converted mechanically into turnings leads to significantly higher product yields in Simmons-Smith and Reformatsky reactions both with (by means of zinc-copper couple formation in the case of Simmons-Smith reactions) and without the abovementioned additional activation methods than does commercially available zinc powder.
As an alternative, the treatment of zinc according to the invention can also be carried out in a molten phase by bringing the zinc melt into contact with metal hydride or metal hydride mixtures. Accordingly, measures which serve to rapidly disperse the metal hydride(s) in the liquid zinc matrix (stirring, blowing inert gases into the melt, etc.) are advantageously used in this variant of the process of the invention.
When the zinc obtained by the above-described treatment process was used in Simmons-Smith reactions carried out by the literature method (R. J. Rawson, I. T. Hanison, J. Org. Chem. 1970, 35, 2057-2058, herein incorporated by reference), new and surprising results were able to be obtained.
A test reaction chosen was, for example, the conversion of cyclohexene into norcarane using CH2I2 as carbene transferer.
The yields of norcarane when using zinc which has been treated with metal hydride and activated with 10% of copper(I) chloride were 5% higher than when using commercial zinc which had likewise been activated with 10% of copper(I) chloride.
As test reaction for comparative reactivity studies in Reformatsky reactions to compare zinc which has been obtained by pretreatment with hydrides and commercially available zinc, the yields in the synthesis of ethyl 3-hydroxybutyrate from ethyl bromoacetate/acetaldehyde and the yields in the synthesis of ethyl 3-hydroxyoctanoate from ethyl bromoacetate/hexanal were determined (Houben-Weyl, Methoden der organischen Chemie, 1973, volume 13/2a, 818). Both batches of zinc are used without any further customary activation.
In the synthesis of ethyl 3-hydroxybutyrate, it has been found that the product yield when using zinc treated with metal hydride is 7% higher than when using the commercial batch of zinc.
A similar trend is found in the synthesis of ethyl 3-hydroxyoctanoate. The product yield was able to be increased by an average of 4% when using hydride-pretreated zinc compared to commercial zinc.
In all investigations according to the present invention on the use of zinc treated with metal hydride in the synthesis of organozinc reagents, it could clearly be seen that significantly higher product yields than when using commercial batches of zinc can be achieved. This is all the more astonishing when one remembers that the reactivity studies presented were carried out using turnings of zinc treated with metal hydride which had a significantly lower available metal surface area in contrast to commercially available zinc powder. Furthermore, the generally accepted view that only the use of a zinc-copper couple in Simmons-Smith reactions leads to high yields of cyclopropanation products was surprisingly able to be clearly disproved. The investigations carried out surprisingly present exactly the opposite picture. Dispensing with the xe2x80x9cactivated metal couplexe2x80x9d even leads to an increased reactivity when zinc treated with metal hydride is used, as the results impressively demonstrate.
It could very clearly be seen that the significantly higher activity of the zinc treated with metal hydride compared to commercial zinc in organometallic synthetic chemistry has a decisive positive effect on the product yield. It is obvious that a similarly distinct positive effect on further important and decisive reaction parameters such as reaction time, reaction temperature, stereoselectivity and regioselectivity, etc., can be achieved.
Viewed purely formally, the Simmons-Smith reaction is, according to the literature (Houben-Weyl, Methoden der organischen Chemie, 1973, volume 13/2a, 838-852, herein incorporated by reference), is an addition reaction involving the organozinc compound CH2X2ZnX which is formed as an intermediate from Zn and CH2X2 (X=Br, I) and can also be isolated and which reacts with an olefin to form a cyclopropane with elimination of ZnX2. Accordingly, there are further possible uses of the zinc according to the present invention in the synthesis and isolation of organozinc compounds such as dialkylzinc, diarylzinc, alkylarylzinc, alkylzinc halide, arylzinc halide, zinc dihydride, alkylzinc hydride, arylzinc hydride, dialkylzinc alkoxide, diarylzinc alkoxide, alkylzinc alkylalkoxide, alkylzinc arylalkoxide, arylzinc alkylalkoxide, arylzinc arylalkoxide, zinc dialkylamide, zinc diarylamide, alkylzinc alkylamide, alkylzinc arylamide, arylzinc alkylamide and arylzinc arylamide compounds.
The invention will now be illustrated by the following examples, without a restriction of its scope being implied. The experiments described below were carried out in an argon atmosphere. Air- and water-free solvents were used. Toluene and diethyl ether were dried over sodium. The organic and inorganic starting materials used were employed without further purification or drying.
The following were used: dibromomethane (99%), methyl trans-crotonate (98%), ethyl bromoacetate (98%), acetaldehyde (99.5%) and hexanal (96%) from Acros; cyclohexene (99%) and 4-vinylcyclohexene (99%) from Aldrich; Diiodomethane, copper(I) chloride (96%) and zinc powder for the reactivity studies from Fisher Scientific (97.2%). For the treatment of zinc with hydrides, use was made of zinc powder from Grillo-Werke AG (99.95%).